The present invention relates to a proximity detector for detecting a substrate through a change of electrostatic capacity.
A proximity detector is placed on the front edge of the door of an elevator apparatus and the like. When the door closes or fully opens, it detects an obstacle on the door travelling path in non-contact manner, to reversely open the door or stop the closing operation of the door.
An exemplary circuit of this kind proximity detector of prior art will be shown in FIG. 1. In the figure, reference numerals (1) and (2) are a pair of antennae disposed longitudinally with a space therebetween along the front edge of the door and have electrostatic capacities (3) and (4) against earth. These antennae (1) and (2) have electrostatic capacities (6) and (7) to a shield plate (5) disposed with insulation from these antennae. The antennae and the shield plate are disposed so as to satisfy the relation, electrostatic capacity (3).times.electrostatic capacity (7)=electrostatic capacity (4).times.electrostatic capacity (6).
Reference numeral (8) is an AC power source to supply voltage to a bridge circuit comprised of the capacities (3) and (4) and capacities (6) and (7). A transformer (9) is used to detect unbalance in the bridge circuit.
Normally, this bridge circuit is balanced and, when a substrate approaches to the antennae (1) or (2), the approach of the substrate is asymmetrically made to the antennae (1) and (2) so that the bridge circuit produces an unbalance output signal. This is detected by a transformer (9), amplified by an AC amplifier (10), rectified by a rectifying circuit (11), and the output of the rectifying circuit is amplified by a DC amplifier (12) to drive a relay (13). Through the operation of the relay (13), closing operation of the door of the elevator is stopped or reversed to prevent the substrate from being nipped by the door.
In this apparatus, the electrostatic capacities (3) and (4) between the antennae (1) and (2), and earth are disposed separately from the capacities (6) and (7) between the antennae (1) and (2), and the shield plate (5). For this, insulation must be doubly made for the antennae (1) and (2) and earth; one insulation is made between earth and the shield plate and another between the shield plate (5) and the antennae (1) and (2).
During closing of the door, if one desires a sufficiently high sensitivity, the output voltage of the AC power source (8) must be large in order to prevent erroneous operation or sensitivity reduction due to characteristic change of parts used in the amplifier and the like.
Even if the capacitances of the electrostatic capacities (3), (4), (6) and (7) forming respective bridge arms of the bridge circuit are balanced, a slight dielectric loss tangent (tan .delta.) produces a phase difference of the voltage induced in the antennae (1) and (2), and a phase unbalance voltage appears at the output of the detecting transformer (9). For this, it is difficult to sufficiently increase the sensitivity of detection during the door closing.
With a view of overcoming the above-mentioned disadvantages, the present invention has an object to provide a proximity detector with a simplified assemblage of the related parts of the antennae, elimination of the phase unbalance voltage and high sensitivity.